Stress can make it difficult to focus, and being unable to focus can cause stress in our field. So, there's a positive feedback which is horrible to experience.

Having experienced this myself recently (a few months ago, for about a month acutely), the solution that worked for me is to cut the cycle at the "inability to focus causes stress" part. Just accept it - right now, you can't focus, and that's okay. The second part of my solution was to remove all distraction, and for periods of work I allowed myself to not focus on work, but I also agreed (with myself) to not allow myself to focus on other things, either. Eventually, a combination of stress reduction and simple boredom got me working productively again. :)

I've experienced this before, and last time I used medication (Adderol) to fix it. I think that's a valid way to break the cycle too, although I would recommend the above procedure first.

Note: This is a special application of the more general observation that most suffering comes from feeling bad about feeling bad, not the original feeling bad part. So, if you can stop feel bad about feeling bad, you don't have to worry about the first-order feeling - it will go away on it's own quickly, because feelings naturally decay pretty quickly. Also, if you want to talk to someone you can contact me via my profile.